Fast packet-switching, known also as label-addressed switching or asynchronous time-division switching, is a digital information switching technique wherein information blocks, which are associated with a label characterizing the information and which arrive at random instant at the switching devices, are switched solely on the base of the label contents. This technique allows considerably improved performance by comparison with conventional packet switching, particularly with respect to protocol simplicity, information processing speed and flexibility. For these reasons it is the most promising switching technique for implementing a broadband integrated service network where the bandwidth requirements of the individual communications are met while respecting their time constraints, and the information is switched independently of the service type.
In implementing a broadband network, the use of optical technology appears an obvious solution, and it is desirable to use such technology not only for transmission between network nodes, but also for switching in the nodes. Thus all optical-to-electrical conversions and vice versa can be dispensed with.
A totally optical fast packet-switching network requires optical switching matrices and optical processing means to carry out the operations relevant to node management and, more particularly, signalling handling and routing.
Optical switching matrices to be used in time-division or space-division circuit-switching systems are largely described in the literature. For instance, the paper "Photonic switching using direction couplers" by H. S. Hinton, IEEE Communications Magazine, Vol. 25, No. 5, May 1987, describes optical switching matrices based on switching elements with two inputs and two outputs. These elements can be used also for implementing self-routing switching matrices, e.g. with Omega or Delta topology better suited to fast packet switching, as described in the paper "Access and Alignment of Data in an Array Processor" by D. H. Lawrie, IEEE Transactions on Computers, Vol. C-25, Dec. 1975, pages 1145 and ff.
By contrast optical processing means having the processing capacity necessary for managing the nodes of a network of the type considered do not yet exist, even though at least relatively simple processing means, capable of controlling the routing through the optical switching matrix, might be available in the future. Therefore, at the present time, the most realistic solution to the problem of introducing the optical technology into a fast packet-switching network seems the use of optical fiber connections between the nodes and of electrically-controlled optical switching matrices within the nodes.